The F-111 pioneered several technologies for production aircraft, including variable-sweep wings, afterburningturbofan engines, and automated terrain-following radar for low-level, high-speed flight. Its design influenced later variable-sweep wing aircraft, and some of its advanced features have since become commonplace. The F-111 suffered a variety of problems during initial development and several of its intended roles, such as naval interception, with the F-111B, failed to materialize.

USAF F-111 variants were retired in the 1990s, with the F-111Fs in 1996 and EF-111s in 1998. The F-111 has been replaced in USAF service by the F-15E Strike Eagle for medium-range precision strike missions, while the supersonic bomber role has been assumed by the B-1B Lancer. The RAAF was the last operator of the F-111, with its aircraft serving until December 2010.

Development

Early requirements

The U-2 incident of May 1960, in which an American CIAU-2 spy plane was shot down over the USSR, stunned the United States government. Besides greatly damaging Soviet relations, the incident showed that Russia had developed a surface-to-air missile that could reach aircraft above 60,000 feet. The United States Air ForceStrategic Air Command (SAC) and the RAF Bomber Command's plans to send subsonic, high-altitude B-47 and V bomber formations into the USSR were now much less viable.[3] By 1960, SAC had begun moving to low-level penetration. This greatly reduces radar detection distances; and, at that time, SAMs were ineffective against low-flying aircraft, and interceptor aircraft did not have as large a speed advantage at low-level.[4] The Air Force's Tactical Air Command (TAC) was largely concerned with the fighter-bomber and deep strike/interdiction roles. TAC was in the process of receiving its latest design, the Republic F-105 Thunderchief, which was designed to deliver nuclear weapons fast and far, but required long runways.[5] A simpler variable geometry wing configuration with the pivot points further out from the aircraft's centerline was reported by NASA in 1958, which made swing wings viable.[6] This led Air Force leaders to encourage its use.[7] In June 1960, the USAF issued specification SOR 183 for a long-range interdiction/strike aircraft able to penetrate Soviet air defenses at very low altitudes and high speeds.[8] The specification also called for the aircraft to operate from short, unprepared airstrips.[7]

In the 1950s the United States Navy sought a long-range, high-endurance interceptor aircraft to protect its carrier battle groups against long-range anti-ship missiles launched from Soviet jet bombers and submarines. The Navy needed a fleet air defense (FAD) fighter with a more powerful radar, and longer range missiles than the F-4 Phantom II to intercept both enemy bombers and missiles.[9] Seeking a FAD fighter, the Navy started with the subsonic, straight-winged aircraft, the Douglas F6D Missileer in 1957.[10][page needed] The Missileer was designed to carry six long-range missiles and loiter for five hours, but would be defenseless after firing its missiles.[9] The program was canceled in December 1960.[10][page needed][11] The Navy had tried variable geometry wings with the XF10F Jaguar, but abandoned it in the early 1950s. It was NASA's simplification which made the variable geometry wings practical.[6] By 1960, increases in aircraft weights required improved high-lift devices, such as variable geometry wings.[12][13] Variable geometry offered high speeds, and maneuverability with heavier payloads, long range, and the ability to takeoff and land in shorter distances.[12]

Tactical Fighter Experimental (TFX)

The U.S. Air Force and Navy were both seeking new aircraft when Robert McNamara was appointed U.S. Secretary of Defense in January 1961.[14] The aircraft sought by the two armed services shared the need to carry heavy armament and fuel loads, feature high supersonic speed, twin engines and two seats, and probably use variable geometry wings.[15] On 14 February 1961, McNamara formally directed the services to study the development of a single aircraft that would satisfy both requirements. Early studies indicated that the best option was to base the design on the Air Force requirement, and use a modified version for the Navy.[16] In June 1961, Secretary McNamara ordered the go ahead of Tactical Fighter Experimental (TFX) despite Air Force and the Navy efforts to keep their programs separate.[17]

The USAF and the Navy could only agree on swing-wing, two-seat, twin-engine design features. The USAF wanted a tandem seat aircraft for low level penetration ground-attack, while the Navy wanted a shorter, high altitude interceptor with side-by-side seating to allow the pilot and RIO to share the radar display.[16] Also, the USAF wanted the aircraft designed for 7.33 g with Mach 2.5 speed at altitude and Mach 1.2 speed at low level with an approximate length of 70 ft (21.3 m). The Navy had less strenuous requirements of 6 g with Mach 2 speed at altitude and high subsonic speed (approx. Mach 0.9) at low level with a length of 56 ft (17.1 m). The Navy also wanted the aircraft with a nose large enough for a 48 in (1.2 m) diameter radar dish.[16][18]

McNamara developed a basic set of requirements for TFX based largely on the Air Force's requirements and, on 1 September 1961, ordered the Air Force to develop it.[16][18] A request for proposals (RFP) for the TFX was provided to industry in October 1961. In December, proposals were received from Boeing, General Dynamics, Lockheed, McDonnell, North American and Republic. The evaluation group found all the proposals lacking, but Boeing and General Dynamics were selected to submit enhanced designs. Boeing's proposal was recommended by the selection board in January 1962, with the exception of the engine, which was not considered acceptable. Switching to a crew escape capsule, instead of ejection seats and alterations to radar and missile storage were also needed. Both companies provided updated proposals in April 1962. Air Force reviewers favored Boeing's offering, while the Navy found both submissions unacceptable for its operations. Two more rounds of updates to the proposals were conducted, with Boeing being picked by the selection board.[19]

In November 1962, McNamara selected General Dynamics' proposal due to its greater commonality between Air Force and Navy versions. The Boeing aircraft shared less than half of the major structural components. General Dynamics signed the TFX contract in December 1962. A Congressional investigation followed, but could not change the selection.[19]

Design phase

The F-111A and B variants used the same airframe structural components and TF30-P-1 turbofan engines. They featured side by side crew seating in an escape capsule as required by the Navy. The F-111B's nose was 8.5 feet (2.59 m) shorter so as to fit on existing carrier elevator decks, and had 3.5 feet (1.07 m) longer wingtips to improve on-station endurance time. The Navy version would carry an AN/AWG-9Pulse-Doppler radar and AIM-54 Phoenix missiles. The Air Force version would carry the AN/APQ-113 attack radar and the AN/APQ-110 terrain-following radar and air-to-ground armament.[20] A team of engineers at General Dynamics was led by Robert H. Widmer.[21]

Lacking experience with carrier-based fighters, General Dynamics teamed with Grumman for the assembly and testing of the F-111B aircraft. In addition, Grumman would also build the F-111A's aft fuselage and the landing gear.[22] The General Dynamics and Grumman team faced ambitious requirements for range, weapons load, and aircraft weight.[23] The F-111 design also included new features on a production military aircraft, such as variable-geometry wings and afterburning turbofan engines.[22]

The F-111A mock-up was inspected in September 1963. The first test F-111A was rolled out of Plant 4, General Dynamics' Fort Worth, Texas plant, on 15 October 1964. It was powered by YTF30-P-1 turbofans and used a set of ejector seats as the escape capsule was not yet available.[20] The F-111A first flew on 21 December 1964 from Carswell Air Force Base, Texas.[24] The first F-111B was also equipped with ejector seats and first flew on 18 May 1965.[25][26]

To address stall issues in certain parts of the flight regime, the engine inlet design was modified in 1965-66, ending with the "Triple Plow I" and "Triple Plow II" designs.[27] The F-111A achieved a speed of Mach 1.3 in February 1965 with an interim intake design.[20][27] Cracks in the F-111's wing attach points were first discovered in 1968 during ground fatigue testing - an F-111 crashed the following year due to this issue. The attach structure required redesign and testing to ensure adequate design and workmanship.[28] Flight testing of the F-111A ran through 1973.[29]

The F-111B was canceled by the Navy in 1968 due to weight and performance issues, along with the need for additional fighter requirements.[30][31] The F-111C model was developed for Australia. Subsequently, the improved F-111E, F-111D, F-111F models were developed for the US Air Force. The strategic bomber FB-111A and the EF-111 electronic warfare versions were later developed for the USAF.[32] Production ended in 1976[33] after 563 F-111 aircraft were built.[1]

Design

Overview

The F-111 was an all-weather attack aircraft, capable of low-level penetration of enemy defenses to deliver ordnance on the target.[34] The F-111 featured variable geometry wings, an internal weapons bay and a cockpit with side by side seating. The cockpit was part of an escape crew capsule.[35] The wing sweep varied between 16 degrees and 72.5 degrees (full forward to full sweep). The wing included leading edge slats and double slotted flaps over its full length.[36] The airframe was made up mostly of aluminum alloys with steel, titanium and other materials used in places.[37] The fuselage was made of a semi-monocoque structure with stiffened panels and honeycomb sandwich panels for skin.[36][37]

The F-111 used a three-point landing gear arrangement, with a two-wheel nose gear and two single-wheel main landing gear. The landing gear door for the main gear, which was positioned in the center of the fuselage, also served as a speed brake in flight.[36][38] Most F-111 variants included a terrain-following radar system connected to the autopilot. The aircraft was powered by two Pratt & Whitney TF30 afterburning turbofan engines. The F-111's variable geometry wings, escape capsule, terrain following radar, and afterburning turbofans were new technologies for production aircraft.[39]

Armament

Weapons bay

The F-111 featured an internal weapons bay that could carry bombs, a removable 20 mm M61 cannon, or auxiliary fuel tanks.[40][41] For bombs, the bay could hold two 750 lb (340 kg) M117 conventional bombs, one nuclear bomb or practice bombs. The F-111B was to carry two AIM-54 Phoenix long-range air-to-air missiles in the bay. The cannon had a large 2,084-round ammunition tank, and its muzzle was covered by a fairing; however, it was rarely fitted on F-111s.[42]

The FB-111 could carry two AGM-69 SRAM air-to-surface nuclear missiles in its weapons bay.[45] General Dynamics trialed an arrangement with two AIM-9 Sidewinder air-to-air missiles carried on rails in a trapeze arrangement from the bay, but this was not adopted.[42] Early F-111 models had radars equipped to guide the AIM-7 Sparrow medium-range air-to-air missile, but it was never fitted.[46]

External ordnance

Each wing was equipped for four underwing pylons. The inner two pylons on each wing would rotate to align with the fuselage, while the outer two were fixed. Each pylon had a capacity of 5,000 pounds (2,300 kg). Various bombs and missiles could be carried on the pylons. Auxiliary fuel drop tanks with 600 US gallons (2,300 L) capacity each could be fitted.[41]

The design of the F-111's fuselage prevents the carriage of external weapons under the fuselage. But two stations are available on the underside for electronic countermeasures (ECM) pods and/or datalink pods; one station is on the weapon bay, and the other on the rear fuselage between the engines.[46] The F-111's maximum practical weapons load was limited, since the fixed pylons could not be used with the wings fully swept.[47]

Tactical F-111s were fitted with shoulder rails on the four inner swiveling pylons to mount AIM-9 Sidewinder air-to-air missiles for self-defense.[42] Australian F-111Cs were equipped to launch the Harpoon anti-ship missile, and the Popeye stand-off missile.[48] FB-111As could carry the same conventional ordnance as the tactical variants, but their wing pylons were more commonly used for either fuel tanks or strategic nuclear gravity bombs. They could carry up to four AGM-69 SRAM nuclear missiles on the pylons.[49]

After early testing, a detachment of six aircraft were sent in March 1968 to Southeast Asia for Combat Lancer testing in real combat conditions in Vietnam. In little over a month, three aircraft were lost and the combat tests were halted. It turned out that all three had been lost through a malfunction in the horizontal stabilizer, not by enemy action.[53] This caused a storm of criticism in the U.S. It was not until 1971 that 474 TFW was fully operational.[54]

September 1972 saw the F-111 back in Southeast Asia, stationed at Takhli Air Base, Thailand. F-111As from Nellis AFB participated in the final month of Operation Linebacker and later the Operation Linebacker II aerial offensive against the North Vietnamese. F-111 missions did not require tankers or ECM support, and they could operate in weather that grounded most other aircraft. One F-111 could carry the bomb load of four McDonnell Douglas F-4 Phantom IIs. The worth of the new aircraft was beginning to show; F-111s flew more than 4,000 combat missions in Vietnam with only six combat losses.[55]

From July 1974 until June 1975, F-111As of the 428th and 429th Tactical Fighter Squadrons were stationed at Korat Royal Thai Air Force Base. They performed missions from there to Cambodia, Laos and other areas in Southeast Asia against Communist forces attempting to oust Cambodia's government.[56]

On 14 April 1986, 18 F-111s and approximately 25 Navy aircraft conducted air strikes against Libya under Operation El Dorado Canyon. The 18 F-111s of the 48th Tactical Fighter Wing and the 20th Tactical Fighter Wing flew what turned out to be the longest fighter combat mission in history.[57] The round-trip flight between RAF Lakenheath/RAF Upper Heyford, United Kingdom and Libya of 6,400 miles (10,300 km) spanned 13 hours. One F-111 was lost over Libya, probably shot down.[57]

F-111s participated in the Gulf War (Operation Desert Storm) in 1991. During Desert Storm, F-111Fs completed 3.2 successful strike missions for every unsuccessful one, better than any other U.S. strike aircraft used in the operation.[58] The group of 66 F-111Fs dropped almost 80% of the war's laser-guided bombs, including the GBU-15 and the penetrating, bunker-buster GBU-28.[59] Eighteen F-111Es were also deployed during the operation.[58][60] The F-111s were credited with destroying more than 1,500 Iraqi tanks and armored vehicles.[60] Their use in the anti-armor role was dubbed "tank-plinking".[61]

The F-111[N 1] was in service with the USAF from 1967 through 1998. The Strategic Air Command had FB-111s in service from 1969 through 1992. At a ceremony marking the F-111's USAF retirement, on 27 July 1996, it was officially named Aardvark, its long-standing unofficial name.[63] The USAF retired the EF-111 variant in 1998.[64]

Variants

F-111A

The F-111A was the initial production version of the F-111. Early A-models used the TF30-P-1 engine. Most A-models used the TF30-P-3 engine with 12,000 lbf (53 kN) dry and 18,500 lbf (82 kN) afterburning thrust[20] and "Triple Plow I" variable intakes, providing a maximum speed of Mach 2.3 (1,450 mph, 2,300 km/h) at altitude.[65] The variant had a maximum takeoff weight of 92,500 lb (42,000 kg) and an empty weight of 45,200 lb (20,500 kg).[66]

Total production of the F-111As was 158, including 17 pre-production aircraft that were later brought up to production standards.[68] 42 F-111As were converted to EF-111A Ravens for an electronic warfare tactical electronic jamming role.[69] In 1982, four surviving F-111As were provided to Australia as attrition replacements and modified to F-111C standard; these were fitted with the longer-span wings and reinforced landing gear of the C-model.[70]

F-111B

Main article: General Dynamics-Grumman F-111B

The F-111B was to be a fleet air defense (FAD) fighter for the U.S. Navy, fulfilling a naval requirement for a carrier-based fighter aircraft capable of carrying heavy, long-range missiles to defend aircraft carriers and their battle groups from Soviet bombers and fighter-bombers equipped with anti-ship missiles. General Dynamics, lacking experience with carrier-based aircraft, partnered with Grumman for this version. F-111B suffered development issues and Navy requirements changed to an aircraft with maneuverability for dogfighting. The swing-wing configuration, TF-30 engines, AIM-54 Phoenix air-to-air missiles and AWG-9 radar developed for this aircraft were used on its replacement, the Grumman F-14 Tomcat. The Tomcat would be large enough to carry the AWG-9 and Phoenix weapons system while exceeding the F-4's maneuverability.[73]

F-111C

The F-111C is the export version for Australia, combining the F-111A with longer F-111B wings and strengthened FB-111A landing gear. Australia ordered 24 F-111s and, following delays, the Royal Australian Air Force accepted the aircraft in 1973.[74] Four were converted to the RF-111C reconnaissance variant in 1979-80. Australia also purchased four ex-USAF F-111As and converted them to C standard. F-111C aircraft received avionics, weapons system and other upgrades during their time in service.[70][75] The RAAF retired its last F-111Cs in December 2010.[76]

F-111D

The F-111D was an upgraded F-111A equipped with newer Mark II avionics, more powerful engines, improved intake geometry, and an early glass cockpit. The variant was first ordered in 1967 and delivered from 1970-73. The F-111D reached initial operational capability in 1972. Deliveries were delayed due to avionics issues. 96 F-111Ds were built.[77] The sole operator of this variant was the 27th TFW stationed at Cannon AFB, New Mexico.

The F-111D used the new Triple Plow II intakes, which were located four inches (100 mm) further away from the airframe to prevent engine ingestion of the sluggish boundary layer air that was known to cause stalls in the TF30 turbofans. It had more powerful TF30-P-9 engines with 12,000 lbf (53 kN) dry and 18,500 lbf (82 kN) afterburning thrust.[78]

The Mark II avionics were digitally integrated microprocessor systems, some of the first used by the USAF, offering tremendous capability, but substantial problems. The Rockwell Autonetics digital bombing-navigation system included inertial navigation system, AN/APQ-130 attack radar system and Doppler radar. It also included digital computer set and multi-function displays (MFDs). The terrain-following radar was the Sperry AN/APQ-128.[79] The attack radar featured a Doppler beam-sharpening, moving target indicator (MTI), and continuous beam for guiding semi-active radar homing missiles.[44][80]

It took years to improve the reliability of the avionics, but issues were never fully addressed.[44][77] The F-111D was withdrawn from service in 1991 and 1992.[81]

F-111E

The F-111E was a simplified, interim variant ordered after the F-111D was delayed. The F-111E used the Triple Plow II intakes, but retained the F-111A's TF30-P-3 engines and Mark I avionics.[82] The weapon stores management system was improved and other small changes made.[83]

The E-model was first ordered in 1968 and delivered from 1969-71. It achieved initial operational capability in 1969.[84] The variant's first flight occurred on 20 August 1969. 94 F-111Es were built.[83] Some F-111Es were based in the UK until 1991. The avionics were upgraded on some E-models as part of an Avionics Modernization Program. The variant served in 1990-91 during the Gulf War. Some F-111Es received improved TF30-P-109 engines in the early 1990s. All F-111Es were retired to AMARC by 1995.[85]

F-111F

The F-111F was the final F-111 variant produced for Tactical Air Command, with a modern, but less expensive, Mark IIB avionics system.[86] The USAF approved development of the variant in 1969. It also included the more powerful TF30-P-100 engine and strengthened wing carry through box. 106 were produced between 1970 and 1976.[87]

The F-111F's Mark IIB avionics suite used a simplified version of the FB-111A's radar, the AN/APQ-144, lacking some of the strategic bomber's operating modes but adding a new 2.5 mi (4.0 km) display ring. Although it was tested with digital moving-target indicator (MTI) capacity, it was not used in production sets.[86] The Mark IIB avionics combined some Mark II components with FB-111A components, such as the AN/APQ-146 terrain-following radar. The F-111E's weapon management system was also included.[88]

The F-model used the Triple Plow II intakes, along with the substantially more powerful TF30-P-100 turbofan with 25,100 lbf (112 kN) afterburning thrust. An adjustable engine nozzle was added to decrease drag.[87] The P-100 engine greatly improved the F-111F's performance.[89] The engines were upgraded to the TF30-P-109 version,[90] later in the 1985–86 timeframe.

In the early 1980s, the F-111F began to be equipped with the AVQ-26 Pave Tack forward looking infrared (FLIR) and laser designator system, which provided for the delivery of precision laser-guided munitions and was mounted in the internal weapons bay.[91] The Pacer Strike avionics update program replaced analog equipment with new digital equipment and multi-function displays.[92] The last USAF F-111s were withdrawn from service in 1996, replaced by the McDonnell Douglas F-15E Strike Eagle.[93]

F-111K

Main article: General Dynamics F-111K

The British government canceled the BAC TSR-2 strike aircraft in 1965, citing the lower costs for the TFX and ordered 50 F-111K aircraft in February 1967 for the Royal Air Force.[94] The F-111K was to be supplemented later by the Anglo-French Variable Geometry Aircraft then under development. The F-111K was based on the F-111A with longer F-111B wings,[94] FB-111 landing gear, Mark II navigation/fire control system, and British supplied mission systems. Other changes included weapons bay modifications, addition of a centerline pylon, a retractable refueling probe, provisions for a reconnaissance pallet, and a higher gross weight with the use of FB-111A landing gear.[95]

In January 1968, the UK terminated its F-111K order,[95] citing higher cost; increased costs along with devaluation of the pound had raised the cost to around £3 million each.[96] The first two F-111Ks (one strike/recon F-111K and one trainer/strike TF-111K) were in the final stages of assembly when the order was canceled.[95] The two aircraft were later completed and accepted by the USAF as test aircraft with the YF-111A designation.[94]

FB-111A/F-111G

The FB-111A was a strategic bomber version of the F-111 for the USAF Strategic Air Command. With Air Force's Advanced Manned Strategic Aircraft program proceeding slowly, and concerns of fatigue failures in the B-52 fleet, the service needed an interim bomber quickly. The FB-111A was selected in 1965 to replace the supersonic Convair B-58 Hustler and early B-52 variants.[97] The Air Force signed a contract for the FB-111A in 1966. In 1968, plans called for 263 FB-111s, but the total was cut to 76 in 1969. The first production aircraft flew in 1968. Deliveries ended in June 1971.[98]

When the United Kingdom canceled its order for the F-111K in 1968, components for the 48 F-111Ks in manufacturing were diverted to FB-111A production.[99][100] The FB-111A featured longer F-111B wings for greater range and load-carrying ability. The bomber variant was lengthened 2 ft 1 in (63 cm) over the F-111A.[101] Its fuel capacity was increased by 585 gallons (2,214 L) and it had stronger landing gear to compensate for the higher maximum takeoff weight of 119,250 lb (54,105 kg). All but the first aircraft had the Triple Plow II intakes and the TF30-P-7 with 12,500 lbf (56 kN) dry and 20,350 lbf (90 kN) afterburning thrust.[102]

The FB-111A had new electronics, known as the SAC Mark IIB avionics suite. For the FB-111A the system used an attack radar improved from the F-111A's system, along with components that would be used on the F-111D, including the inertial navigation system, digital computers, and multi-function displays.[103] Armament for the strategic bombing role was the Boeing AGM-69 SRAM (short-range attack missile); two could be carried in the internal weapons bay and four more on the inner underwing pylons. Nuclear gravity bombs were also typical FB armament. Fuel tanks were often carried on the third non-swivelling pylon of each wing. The FB-111A had a total weapon load of 35,500 lb (16,100 kg).[102]

Multiple advanced FB-111 strategic bomber designs were proposed by General Dynamics in the 1970s. The first design, referred to as "FB-111G" within the company,[104] was a larger aircraft with more powerful engines with more payload and range. The next was a lengthened "FB-111H" that featured more powerful General Electric F101 turbofan engines, a 12 ft 8.5 in longer fuselage and redesigned, fixed intakes. The rear landing gear were moved outward so armament could be carried on the fuselage there. The FB-111H was offered as an alternative to the B-1A in 1975.[104][105] The similar FB-111B/C was offered in 1979 without success.[106]

The FB-111A became surplus to SAC's needs after the introduction of the B-1B Lancer. The remaining FB-111s were subsequently reconfigured for tactical use and redesignated F-111G. The conversions began in 1989 and ended after 34 F-111G conversions were completed. With the disestablishment of SAC, the FB-111As and F-111Gs were transferred to the newly established Air Combat Command (ACC). They were used primarily for training.[107] The remaining FB-111As were retired in 1991 and the F-111Gs were retired in 1993.[108] Australia bought 15 F-111Gs in 1993 to supplement its F-111Cs.[108] They were retired in 2007.

EF-111A Raven

To replace the aging Douglas EB-66, the USAF contracted with Grumman in 1972 to convert 42 existing F-111As into electronic warfare aircraft. The EF-111A can be distinguished from the F-111A by the equipment bulge atop their tails. In May 1998, the USAF withdrew the final EF-111As from service, placing them in storage at Aerospace Maintenance and Regeneration Center (AMARC).[109]

Popular culture

American artist James Rosenquist immortalized the aircraft in his acclaimed 1965 room-sized pop art painting entitled F-111 that features an early natural-finish example of the aircraft in USAF markings. The painting hangs in the Museum of Modern Art in New York City.[155]

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